As a rather casual plant enthusiast:

*looks at title*

*looks at first image*

Do something about what exactly?

It's interesting that it's a middle leaf. Our orchids tend to grow new leaves on the top and drop leaves on the bottom as they put out new roots going up their stem. Every few years I have to unpot them and cut the bottom few inches, otherwise they will climb out of their pot.

When our orchids drop leaves they look very similar to the leaf on your plant.

In my experience, this variety of orchid thrives on benign neglect. Ours very rarely get fertilized. They also only get watered every two weeks or so and when they get watered the bark they're in is pretty well saturated.

My understanding is that leaves don't "go back." Once a leaf starts yellowing, the best case scenario is that you have a yellow leaf. You can correct deficiencies or overdoses for future leaves, and prevent existing leaves from turning, but you can't fix the ones that have already changed.

Are the clay pebbles new for this plant? I often understood that the older roots from before semi hydroponic will die in those conditions and new roots will grow and be adapted to those same conditions. I used a similar setup with no fertilizer at all and got orchids to flower for a few years, the growth was just a bit smaller every years. They need way less fertilizer than you think : because they grow as epiphytes in the wild without access to soil. Intermittent very low level of fertilizer might be better especially if the leaves are showing sign of a mismatch between root surface and leaves exposed surface (to drying that can be helped with higher air humidity)

I would prepare yourself to lose a few of the older leaves no matter what. Partly that’s just what phalenopsis do, but this one is also healing and adapting to a new situation.

And chill out really hard on the fertilizer. It’s very easy to over do it. I use a weak nutrient solution called “Orchid plant food mist” by miracle grow maybe one good pump on the exposed roots and foliage once a month has been plenty to get annual flower stalks.

It looks quite healthy but the transition from hydro to not-hydro (geoponics, i guess?) will trigger a lot of physiological changes and internal reshuffling of nutrients. It’s inevitable.

There's too much nitrogen in the substrate, going by leaf discoloration. I would exchange half the water, so you don't lower the level so much that it shocks it.